Discharge tube



Sept. 6, 1938.

H. G. BOUMEESTER DISCHARGE TUBE Filed Dec. 28, 1936 I fli oameasier fl/ 46% ,Mc M% Patented Sept. 6, 1938 DYISCHARGE TUBE Huibert Gerard Boumeester, Eindhoven, Netherlands, assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands Application December 28, 1936, Serial No. 117,965 In Germany December 30, 1935 I 50mins, (01. 250-215) My invention relates to discharge tubes whose envelopes comprise a metal portion.

In tubes of such a construction; which are frequently used for both grid controlled and uncontrolledrectifiers, it isdesirable to screen the discharge path, and also in some casesoone or more of the electrodes, from external influences. This has been effected in various manners, for example by screwing a metal screening member to the metal envelope portion, .orrb ylapplying a conductive coating upon either the inner or outer surface of the envelope. The former method had the disadvantage that the screw threads became distorted when the glass envelope portions were sealed to the metal portion, whereas external coatings had the disadvantage that the insulating wall of the envelope reduced the screening effect.

Although internal coatings eliminated such reduction in the screening effect by the tube wall, it was very difficult toprovide a suitable electrical connection. It has been proposed to connect such coatings to a lead sealed in the envelope, by means of a resilient ring connected to the lead and bearing upon the coating, or by means of a pool of liquid mercury; however such an electric contact with the coating was often defective or unreliable. Furthermore, the use of an additional lead through the envelope complicated and increased the cost of the tube.

The object of my invention is to eliminate the above difiiculties and to provide a simple, inexpensive and effective screen for such tubes.

In accordance with the invention, I provide a conductive coating upon the inner surface of the envelope and extend this coating partly over the metal envelope portion so as to make a good electrical connection therewith.

Such a construction has the advantage that 40 the conductive coating may be provided in a simple and inexpensive manner, and that no additional connections between the coating and a supply lead are necessary. Furthermore, the metal envelope portion, which may serve as a control or ignition electrode, also serves as the supply lead to the coating, thereby simplifying the construction.

Further advantages of my invention will appear as the description progresses.

In order that my invention may be clearly understood and readily carried into efiect, I shall describe same more fully with reference to the accompanying drawing, in which the single figure is a sectional view of a grid-controlled recti- 55 fier tube.

The tube shown in the drawing has an envelope comprising a glass cathode portion l, a glass anode portion 2, and an annular metal portion 3 provided with a cross shaped radial partition l5 and hermetically sealed between the glass portions. The metal portion 3 serves as a control electrode and has secured to it by means of a clamp ring M, an external lead ill. The filling of the tube may consist of one or more gases, vapors or a mixture thereof. In the present instance, a drop I I of mercuryis arranged in the lower part of the vessel to provide a mercury vapor filling.

. Hermetically sealed in the bottom of portion I are two chrome iron plates l2, to which are secured two conductive supports 4, carrying on their ends an incandescible cathode 5. The supports 4, are connected through the plates l2 to external supply leads l3 secured to plates I 2, and are supported at intermediate points by two cross members insulated by glass beads 8. Within the portion 2 and supported by a conducting rod 6 carried by a plate l2 sealed in the envelope, is an anode 1.

Upon the inner surface of the upper portion of the glass portion l and extending over the inner surface of metal portion 3, is a layer or coating 9 of conductive material. As the material of the layer 9, I prefer to use colloidal graphite, which can be applied in a simple manner from a commercial aqueous solution, for instance the solution known under the trade name aquadag. Silver is also very suitable for use as the coating as it can be quite readily precipitated upon the glass wall by any of the well-known methods. However, as silver is attacked by mercury such a coating cannot be used alone in tubes provided with a filling containing mercury vapor.

In such cases I provide the silver coating, for example, by an electroplating process, with a protective layer of a metal which amalgamates with mercury only with difficulty. As such a protective layer-indicated by l5 in the drawing-chromium or nickel may be used.

It will be noted that the metal portion 3 serves two functions, namely, as an ignition or control electrode, and as a connection to the conductive coating 9 serving as an electrostatic screen for the cathode 5 and for a portion of the discharge path. Furthermore, an excellent electrical connection is obtained between the coating 9 and the metal portion 3 without additional means, and it is unnecessary to provide an additional connection through the tube wall.

While I have described my invention in connec- 1. A discharge tube comprising an envelope" having a metal portion and a vitreous'portion hermetically sealed together, electrodes Within said envelope, a conductive coating upon part'of the inner surface of said vitreous portion and ex tending over part of the inner surface of said metal portion, and a terminal for said coating connected to the outside of said metal portion.

2. A discharge tube comprising an envelope having a vitreous anode portion, a bell-shaped cathode portion, and an annular metal portion hermeticallys'e'aled between said vitreous portiofis and serving as an electrode, an anode disposed 'within 'sa'id'a'node portion and'a cathode disposed within said cathode portion to form a discharge path, and a conductive layer upon a portion of the innersurface of said cathode portion and 'extending over part of the inner surface of said metal portion,"said coating forming a bell shaped screen surrounding said cathode and the portion of the discharge path between said cathode and metal portion.

3. A discharge tube comprising an envelope having a metal portion and a vitreous portion hermetically sealed together, a filling comprising mercury vapor, electrodes within said envelope, a layer of silver upon part of the inner surface of said vitreous portion and extending over part of the inner surface of said metal portion, and a protective coating covering said silver layer, said coating being of a metal substantially unaffected by mercury.

4. A discharge tube comprising an envelope having a metal portion and a vitreous portion hermetically sealed together, a filling comprising mercury vapor, electrodes within said envelope, a layer of silver upon part of the inner surface "o-fsaid vitreous portion and extending over part protective coating of nickel upon said silver layer.-

I-IUIBERT GERARD BOUMEESTER. 

